MIMO Systems for MANETs Vivek Jain

Outline Antenna System Smart Antenna System Gigabit Wireless Links – Design Challenges in SISO Links Need for MIMO MIMO – Leverages Single and Multi-user MIMO System Open Issues

Antenna Definition An antenna is a circuit element that provides a transition form a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy. Antenna research from Miller & Beasley, 2002

8 Element Equally Spaced Circular Antenna Array Antenna System Phased Array Antenna Incident Wave 2 1 3 Greater the number of elements in the array, the larger its directivity 1 2 3 4 5 6 7 4 d 7 5 6 8 Element Linear Equally Spaced Antenna Array 8 Element Equally Spaced Circular Antenna Array

Antenna Pattern of 7-element uniform equally spaced circular array. Antenna System (Cont.) Plane wave phase delay 1 2 3 4 5 6 7 N N-2 N-1 N-3 … d DOA Estimation phase shifters 1 2 3 4 5 6 7 N N-2 N-1 N-3 … … 1,,k 2,,k 3,,k 4,,k 5,,k 6,,k 7,,k N-3,,k N-2,,k N-1,,k N,,k Beam Formation Beam Forming Technique in which the gain pattern of an adaptive array is steered to a desired direction through either beam steering or null steering signal processing algorithms. Adaptive beam forming algorithms can provide substantial gains (of the order of 10log(M) dB, where M is number of array elements) as compared to omni directional antenna system. Antenna Pattern of 7-element uniform equally spaced circular array.

Smart Antenna System (Cont.) Switched Beam Consists of a set of predefined beams. Allows selection of signal from desired user. Beams have narrow main lobe and small side-lobes. Signals received from side-lobes can be significantly attenuated. Uses a linear RF network, called a Fixed Beam-forming Network (FBN) that combines M antenna elements to form up to M directional beams.

Smart Antenna System (Cont.) Adaptive Array Rely on beam-forming algorithm to steer the main lobe of the beam. Can place nulls to the direction of the interferences. Linearly equally Space (LES) antenna array Ability to change antenna pattern dynamically to adjust to noise, interference, and multipath. Consists of several antenna elements (array) whose signals are processed adaptively by a combining network, the signals received at different antenna elements are multiplied with complex weights and then summed to create a steerable radiation pattern. MIMO – Digital adaptive array at both ends of communication link.

Smart Antenna System (Cont.) Interference 1 top view (horizontal) top view (horizontal) 5 4 6 3 7 2 user 1 interference 8 1 9 16 user 10 15 user 2 11 14 12 Interference 2 13 Adaptive array Switched array Wireless Local Area Networks Cellular Communication Networks Military networks Applications

Roadmap from 1–4 G. 802.11n 802.20 Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004

Gigabit Wireless Links – Design Challenges in SISO Links Requirements High spectral efficiency High bandwidth Can’t exceed 4-6 b/s/Hz in NLOS >250-MHz for 4-6 b/s/Hz in NLOS implies 40 GHz range Transmit power <1W Received SNR <30-35dB Low SINR < 10-20dB Shadowing >6GHz Constraints

LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise Reflection Diffraction Scattering Cannot be eliminated !!!

Need for MIMO High data rate wireless communications for WLANs and home A/V networks. MIMO can provide 1-Gb/s wireless links. Good QoS and range capability in NLOS environments. Wired links can provide 10-Gb/s transmission rate !!! However, transceiver design complexity increases !!!

MIMO – Leverages Array Gain Diversity Gain Increase in average SNR due to coherent combining. Requires channel knowledge of transmitter and receiver. Depends on number of transmit and receive antennas. Diversity Gain Diversity mitigates fading in wireless links. ‘MTMR’ links composing MIMO channel fading independently can lead to MTMR-th order diversity as compared to SISO link. Can be extracted in the absence of channel knowledge at the transmitter by designing suitable transmit signals <- space time coding.

MIMO – Leverages (Cont.) Spatial Multiplexing Gain Transmit independent data signals from individual antennas. Receiver can extract different streams under conducive channel conditions – rich scattering. A linear (in min(MT, MR)) in capacity for no additional power or bandwidth expenditure is obtained. Interference Reduction Differentiation between the spatial signatures of the desired channel and co-channel signals is exploited to reduce interference. Requires knowledge of desired signal’s channel. Allows aggressive frequency reuse and thus increases multi-cell capacity.

MIMO – Leverages (Cont.) Array and diversity gains leads to range extension. where, p is path loss component Array gain increases with number of antennas while diversity gain decreases. Array and diversity gain => Signals transmitted from different antennas are dependent. Spatial multiplexing gain => Signals transmitted from different antennas are independent. Signals are transmitted out from different antenna with equal power, same frequency, same modulation format and in same time slot, separated by spatial signatures.

Single and Multi-user MIMO System Single-user MIMO Spectral efficiency is increased by supporting multiple data streams over spatial channels. Spatial diversity is exploited to enhance the detection performance. Multi-user MIMO MIMO channel is evenly divided and allocated to multiple users. Each user channel has access to the space domain over entire transmission channel and frequency bandwidth. Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004

Open Issues Top-down compatibility and bottom-up feasibility. Cross-layer optimization. Efficient MAC protocol to leverage the properties of MIMO links. Capacity of MIMO links. MIMO in Ad hoc networks. Capacity vs. range extension. Transmission starvation at Receiver.

Questions ??? Source: http://www.airgonetworks.com/pdf/Farpoint Group 2003-242.1 MIMO Comes of Age.pdf

References A. J. Paulraj, D. A. Gore, R. U. Nabar and H. Bolcskei, “An Overview of MIMO Communications – A Key to Gigabit Wireless”, Proc. of the IEEE, vol. 92, no. 2, pp. 198-218, Feb. 2004. Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004 Marwin Sanchez G., “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc Rural-Area Networks” Dissertation, June 2002. [Online] http://www.s3.kth.se/radio/Publication/Pub2002/Sanchez_Lict2002.pdf

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